Motor drives and other power conversion systems operate to convert electrical power from one form to another and may be employed in a variety of applications such as powering an electric motor using power converted from a single or multiphase AC input source. Such power converters are typically constructed using a passive or active rectifier to convert input AC power to an intermediate DC, followed by an active inverter stage that converts the intermediate DC to AC output power to drive a motor, power grid, or other load. Matrix converters provide AC-AC conversion, typically without internal DC storage elements. The matrix converter or rectifier/inverter stages generally include switches actuated through various forms of pulse width modulation (PWM), where the PWM switching states used in the rectifier and inverter may be constructed according to selective harmonic elimination (SHE or SHEPWM) or space vector modulation (SVM or SVPWM) or other PWM techniques.
Current source converter (CSC) type drives use the rectifier to provide a controlled DC current in an intermediate DC link circuit, which is then converted by the inverter into drive currents provided to the load, where the link circuit includes one or more inductances, such as a link choke. Voltage source converters (VSCs) regulate the DC voltage across a capacitance in the intermediate circuit and a voltage source inverter (VSI) generates output waveforms by converting the intermediate DC bus voltage. In a common medium voltage drive configuration, switches of an active rectifier are pulse width modulated according to an SHE scheme, and the inverter stage is controlled by SHEPWM when the output frequency is high, and SVPWM may be used for low inverter output frequencies.
It is often important to control the amount of common mode voltages and currents seen by conversion system components and by the load in motor drives and other power converters. For instance, motors are susceptible to damage or performance degradation caused by appearance of excessive common mode voltages on the motor leads. In voltage source converters, the common mode voltage at the load output is related to the regulated DC link voltage, and thus common mode voltage control techniques have been advanced which carefully select VSI inverter switching patterns to reduce output common mode voltages.
U.S. Pat. No. 7,164,254 to Kerkman et al., issued Jan. 16, 2007 and assigned to the assignee of the present application discloses common mode voltage reduction techniques in which the switching sequence is modified to avoid using the zero vectors in order to reduce common mode voltages in the motor. The entirety of this patent is hereby incorporated by reference as if fully set forth herein.
U.S. Pat. No. 7,106,025 to Yin et al., issued Sep. 12, 2006 and assigned to the assignee of the present application discloses techniques for canceling dead time effects in the algorithm to reduce common mode voltages produced by a three-phase power conversion device in a rectifier/inverter variable frequency drive (VFD), the entirety of which is hereby incorporated by reference as if fully set forth herein.
U.S. Pat. No. 6,819,070 to Kerkman et al., issued Nov. 16, 2004 and assigned to the assignee of the present application discloses inverter switching control techniques to control reflected voltages in AC motor drives, the entirety of which is hereby incorporated by reference as if fully set forth herein.
U.S. Pat. No. 7,034,501 to Thunes et al., issued Apr. 25, 2007 and assigned to the assignee of the present application discloses gate pulse time interval adjustment techniques for mitigating reflected waves in AC motor drives, the entirety of which is hereby incorporated by reference as if fully set forth herein.
Current source converters, however, do not have a DC link with fixed voltage, and common mode control techniques for voltage source converters are generally not effective for addressing common mode voltages in current source converters. Instead, conventional current source converter common mode voltage control is typically accomplished by addition of isolation transformers and/or common mode output filter circuits including common mode capacitors connected to the output motor leads.
With respect to pulse width modulation techniques, SVPWM is generally simpler and easier to control on the inverter side than SHEPWM. However, space vector modulation typically generates more common mode voltage on the link choke than does SHEPWM. As a result, the link choke is typically designed for high saturation levels, for instance, where the choke can be 20 to 30% larger where the inverter uses SVPWM than if SHE is used. This increases the cost, size, and weight of the link choke. Accordingly, there is a continuing need to control or reduce common mode currents and voltages in power conversion systems to allow link chokes and other system components to be better optimized.